1. Field of the Invention
The present invention relates generally to wireless communication systems and more particularly, to media access control in these systems.
2. Description of the Related Art
Smart antennas (part of a communication art also known as spatial processing) have been proposed for various types of communication system applications. Generally, spatial processing promises to improve coverage range, data capacity and reliability of wireless communications systems. A few proprietary wireless communications systems that incorporate spatial processing are under development. These proprietary systems, however, typically lack the broad vendor support and volume deployment of standards-based systems, and they generally carry higher equipment costs.
In contrast, a few standards-based wireless communications systems have been widely deployed but they do not incorporate spatial processing. Compared to the proprietary systems, the standards-based systems generally suffer from limited range, limited capacity and poor link reliability.
The integration of spatial processing into a standards-based system that was not designed to incorporate such processing is a substantial challenge. Spatial processing is intended to optimize communication between a specific transmitter and a specific receiver. However, in a dynamically changing channel (either caused by the movement of the transmitter, the movement of the receiver or the movement of objects that affect signal propagation), spatial processing can have unpredictable effects on the ability of other receivers to receive transmissions that have been spatially processed for reception by a specific receiver.
An exemplary standards-based system utilizes Carrier Sense Media Access/Collision Avoidance (CSMA/CA) in which a number of system mechanisms have been provided to properly manage access to the network. For example, one mechanism involves the basic premise of assessing if the channel is busy prior to initiating a transmission. If a system device wishes to initiate a transmission, it first tests whether the channel is busy by sensing received signal energy. If the channel is not busy for a designated period of time TL, then the device may transmit. If the channel is found to be busy, then the device goes through an algorithmic back-off which, in effect, waits for a random amount of time before trying again.
Upon proper reception of a unicast packet (data packet sent to one respective device) by the designated recipient, that recipient responds with an acknowledgement packet within a designated period of time TS. If no acknowledgement is received, the original transmitter assumes that the packet was not received due to a collision or other bad channel condition and waits for a random amount of time prior to trying to retransmit the lost packet. Generally, TL is greater than TS to give priority to response packets (e.g., acknowledgements) over new transactions (wherein the term “transaction” refers to the process of transmitting a packet and receiving a response).
Additional mechanisms have been added to CSMA/CA systems to improve network performance by minimizing time wasted in improperly received transmissions that were caused by bad channel conditions or collisions. One example of such a mechanism is a channel reservation time (CRT) that is maintained by all members of the network. In such a system, most transmitted packets contain a time value for which the channel must be reserved, in order to complete an entire transaction. All devices not engaged in the transaction but which can properly receive any message with a CRT, use this information to allow the transaction to complete before attempting to access the channel. The use of CRTs enhances network behavior when certain wireless devices cannot hear transmissions from other wireless devices but do communicate with a communication hub. In such scenarios, sensing received signal energy on the channel is not sufficient to determine for how long the channel is in use. Therefore, the CRT creates a virtual channel busy indicator that tells the device that the channel is expected to be busy.
Another mechanism to improve network performance is the use of short channel reservation requests (CRR) that are sent prior to transmitting data. In such an instance, the device transmitter waits the allotted time to ensure that the channel is not busy and then sends a short CRR to the hub requesting the channel for a specific period of time. The hub responds with a clear channel acknowledgement (CCA) indicating that the channel is indeed clear and that the channel is reserved for the transmitting device. The CCA generally reserves the channel (i.e., it contains a CRT) for the expected time required for the entire transaction (including necessary acknowledgements).
The original transmitter then proceeds by transmitting its data packet. These transmissions are separated by a time period (TS) and all of them together comprise a transaction. In this transaction, since the hub is transmitting either a CCA or a CRR, it is assumed that all members of the network are able to receive one of these (with its associated CRT) and therefore will not attempt to transmit until the transaction is complete.
Still another mechanism for improving network performance is the use of packet fragmentation. This allows a transmitting device to break a communication packet down into smaller packets and transmit them separately. In poor channel conditions, this approach has advantages because lost packets will now cause less loss of data, and therefore less time lost to retransmission. Generally, each transmitted packet fragment is acknowledged by the receiver upon proper reception. This allows the transmitter to immediately resend lost packets.
The balancing mechanism to packet fragmentation is packet concatenation. This mechanism allows the transmitter to concatenate short packets together to allow them to be transmitted within one larger transmission. Every acknowledgement and every CRR/CCA add a fixed overhead to the transmission of each packet that they correspond to. Because they are intended for reception by all network members, they are generally sent at the lowest possible data rate (e.g., lowest order of modulation, greatest spreading (in the case of spread spectrum systems), and greatest amount of error correction overhead) allowed in the network so that the added overhead is significant. Fewer packets are lost during good channel conditions with channel reservation. Accordingly, packet concatenation reduces overhead during these system conditions.
In contrast to these CSMA/CA system mechanisms, spatial processing utilizes characteristics of the channel that are unique to the relative location of a transmitter and a receiver to optimize the transmission or reception of signals to or from a particular communication device. This is difficult to implement in the media access control (MAC) protocols of CSMA/CA because they generally do not include prior knowledge of transmission directions (hub to client or client to hub) nor which client will be transmitting.
Additionally, in dynamic channel conditions in which the channel or location of the transmitter or receiver changes over the timeframe of a few packets, optimization of transmit spatial processing requires regular monitoring of the channel/location of the involved transmitter and receiver—usually through the reception process. Therefore, a relatively static database that maintains location and channel condition information is insufficient, as the required information can change and mechanisms in the MAC must be utilized to provide the necessary information for periodic updates.
Although connection-oriented MACs have been provided in some proprietary wireless communications systems (to maintain a connection between a transmitter and a receiver so that beamforming may be initially set and then slowly adjusted to manage spatial processing), these connection-oriented MACs differ substantially from contention-based MACs of CSMA/CA system and, accordingly, they are difficult to integrate into such systems.